The Gatling gun's operation centered on a cyclic multi-barrel design which facilitated cooling and synchronized the firing-reloading sequence. Each barrel fired a single shot when it reached a certain point in the cycle, after which it ejected the spent cartridge, loaded a new round and, in the process, allowed the barrel to cool somewhat. This configuration allowed higher rates of fire to be achieved without the barrel overheating.

The Gatling gun was designed by the American inventor Dr. Richard J. Gatling in 1861 and patented on November 4, 1862.[2][3] Gatling wrote that he created it to reduce the size of armies and so reduce the number of deaths by combat and disease, and to show how futile war is.[4]

Although the first Gatling gun was capable of firing continuously, it required a person to crank it; therefore it was not a true automatic weapon. The Maxim gun, invented and patented in 1883, was the first true fully automatic weapon, making use of the fired projectile's recoil force to reload the weapon. Nonetheless, the Gatling gun represented a huge leap in firearm technology.

Prior to the Gatling gun, the only weapons available to militaries, capable of firing many projectiles in a short space of time, were mass-firing volley weapons like the French Reffye mitrailleuse in 1870–1871, and field cannons firing canister shot, much like a very large shotgun. The latter were widely used during and after the Napoleonic Wars. Although the maximum rate of fire was increased by firing multiple projectiles simultaneously, these weapons still needed to be reloaded after each discharge, which for multi-barrel systems like the mitrailleuse was cumbersome and time-consuming. This negated much of the advantage of their high rate of fire per discharge, making them much less powerful on the battlefield. In comparison, the Gatling gun offered a rapid and continuous rate of fire without having to be manually reloaded by opening the breech.

The original Gatling gun was a field weapon which used multiple rotating barrels turned by a hand crank, and firing loose (no links or belt) metal cartridge ammunition using a gravity feed system from a hopper. The Gatling gun's innovation lay neither in the rotating chamber mechanism, first used by the Puckle gun nearly a century and a half before, nor in the use of multiple barrels to limit overheating (used by the mitrailleuse gun); rather, the innovation was the gravity feed reloading system, which allowed unskilled operators to achieve a relatively high rate of fire of 200 rounds per minute.[3]

The Gatling gun was first used in warfare during the American Civil War. Twelve of the guns were purchased personally by Union commanders and used in the trenches during the siege of Petersburg, Virginia (June 1864 – April 1865).[5] Eight other Gatling guns were fitted on gunboats.[6] The gun was not accepted by the American Army until 1866, but a sales representative of the manufacturing company demonstrated it in combat.[7]

Early multi-barrel guns were approximately the size and weight of artillery pieces, and were often perceived as a replacement for cannons firing grapeshot or canister shot.[7] Gatling guns were even mounted aboard ships. Compared with earlier weapons such as the mitrailleuse, which required manual reloading, the Gatling gun was more reliable and easier to operate, and had a lower, but continuous rate of fire. The large wheels required to move these guns around required a high firing position, which increased the vulnerability of their crews.[7]

Sustained firing of gunpowder cartridges generated a cloud of smoke, making concealment impossible until smokeless powder became available in the late 19th century.[8] When fighting troops of industrialized nations, Gatling guns could be engaged by artillery they could not reach and their crews could be targeted by snipers they could not see.[7]

The Gatling gun was used most successfully to expand European colonial empires by killing warriors of non-industrialized societies mounting massed attacks, including the Matabele, the Zulu, the Bedouins, and the Mahdists.[7] Imperial Russia purchased 400 Gatling guns and used them against Turkmen cavalry and other nomads of central Asia.[9] The Royal Navy used Gatling guns against the Egyptians at Alexandria in 1882.[8]

Because of infighting within army ordnance, Gatling guns were again used by the U.S. Army during the Spanish–American War.[10] A four-gun battery of Model 1895 ten-barrel Gatling guns in .30 Army made by Colt's Arms Company was formed into a separate detachment led by Lt. John "Gatling Gun" Parker.[11] The detachment proved very effective, supporting the advance of American forces at the Battle of San Juan Hill, where three of the Gatlings with swivel mountings were used with great success against the Spanish defenders.[12] During the American charge up San Juan and Kettle hills, the three guns fired a total of 18,000 .30 Army rounds in 8 1/2 minutes (an average of over 700 rounds per minute per gun) against Spanish troop positions along the crest of both hills, wreaking terrible carnage.[12]

Despite this remarkable achievement, the Gatling's weight and cumbersome artillery carriage hindered its ability to keep up with infantry forces over difficult ground, particularly in Cuba, where roads were often little more than jungle footpaths. By this time, the U.S. Marines had been issued the modern tripod-mounted M1895 Colt–Browning machine gun using the 6mm Lee Navy round, which they employed to defeat the Spanish infantry at the battle of Cuzco Wells.

The Gatling gun was hand-crank operated with six barrels revolving around a central shaft, although some models had as many as ten. Early models had a fibrous matting stuffed in among the barrels which could be soaked with water to cool the barrels down. Later models eliminated the matting-filled barrels as being counterproductive.

The ammunition was initially a steel cylinder charged with black powder and primed with a percussion cap, because self-contained brass cartridges had not yet been fully developed and available. The shells were gravity-fed into the breech through a hopper or stick magazine on top of the gun. Each barrel had its own firing mechanism. After 1861, new brass cartridges similar to modern cartridges replaced the paper cartridge, but Gatling did not switch to them immediately.

The Model 1881 was designed to use the 'Bruce'-style feed system (U.S. Patents 247,158 and 343,532) that accepted two rows of .45-70 cartridges. While one row was being fed into the gun, the other could be reloaded, thus allowing sustained fire. The final gun required four operators. By 1876, the gun had a theoretical rate of fire of 1,200 rounds per minute which is 20 rounds per second, although 400 rounds per minute was more likely in combat.

Each barrel fires once per revolution at about the same position. The barrels, a carrier, and a lock cylinder were separate and all mounted on a solid plate revolving around a central shaft, mounted on an oblong fixed frame. The carrier was grooved and the lock cylinder was drilled with holes corresponding to the barrels. Each barrel had a single lock, working in the lock cylinder on a line with the barrel. The lock cylinder was encased and joined to the frame. The casing was partitioned, and through this opening the barrel shaft was journaled. In front of the casing was a cam with spiral surfaces. The cam imparted a reciprocating motion to the locks when the gun rotated. Also in the casing was a cocking ring with projections to cock and fire the gun.

Turning the crank rotated the shaft. Cartridges, held in a hopper, dropped individually into the grooves of the carrier. The lock was simultaneously forced by the cam to move forward and load the cartridge, and when the cam was at its highest point, the cocking ring freed the lock and fired the cartridge. After the cartridge was fired the continuing action of the cam drew back the lock bringing with it the spent cartridge which then dropped to the ground.

The grouped barrel concept had been explored by inventors since the 18th century, but poor engineering and the lack of a unitary cartridge made previous designs unsuccessful. The initial Gatling gun design used self-contained, reloadable steel cylinders with a chamber holding a ball and black-powder charge, and a percussion cap on one end. As the barrels rotated, these steel cylinders dropped into place, were fired, and were then ejected from the gun. The innovative features of the Gatling gun were its independent firing mechanism for each barrel and the simultaneous action of the locks, barrels, carrier and breech.

The smallest-caliber gun also had a Broadwell drum feed in place of the curved magazine of the other guns. The drum, named after L. W. Broadwell, an agent for Gatling's company, comprised twenty stick magazines arranged around a central axis, like the spokes of a wheel, each holding twenty cartridges with the bullet noses oriented toward the central axis. This invention was patented in U. S. 110,338. As each magazine emptied, the drum was manually rotated to bring a new magazine into use until all 400 rounds had been fired.

By 1893, the Gatling was adapted to take the new .30 Army smokeless cartridge. The new M1893 guns featured six barrels, and were capable of a maximum (initial) rate of fire of 800–900 rounds per minute. Dr. Gatling later used examples of the M1893 powered by electric motor and belt to drive the crank.[13] Tests demonstrated the electric Gatling could fire bursts of up to 1,500 rpm.

The M1893, with minor revisions, became the M1895, and 94 guns were produced for the U.S. Army by Colt. Four M1895 Gatlings under Lt. John H. Parker saw considerable combat during the Santiago campaign in Cuba in 1898. The M1895 was designed to accept only the Bruce feeder. All previous models were unpainted, but the M1895 was painted olive drab (O.D.) green, with some parts left blued.

The Model 1900 was very similar to the model 1895, but with only a few components finished in O.D. green. The U.S. Army purchased a quantity of M1900s. All Gatling Models 1895–1903 could be mounted on an armored field carriage. In 1903, the Army converted its M1900 guns in .30 Army to fit the new .30-03 cartridge (standardized for the M1903 Springfield rifle) as the M1903. The later M1903-'06 was an M1903 converted to .30-06. This conversion was principally carried out at the Army's Springfield Armory arsenal repair shops. All models of Gatling guns were declared obsolete by the U.S. Army in 1911, after 45 years of service.[14]

After the Gatling gun was replaced in service by newer recoil- or gas-operated weapons, the approach of using multiple externally powered rotating barrels fell into disuse for many decades. However, some examples were developed during the interwar years, but only existed as prototypes, or were rarely used. The concept resurfaced after World War II with the development of the M61 Vulcan. Many other versions of the Gatling gun were built in the late 20th century to the present. An example is the M61 Vulcan, built in 1956.